Generation and characterization of a murine model for Hereditary Spastic Paraplegia SPG11

Hereditary spastic paraplegias (HSPs) are clinically and genetically heterogenous group of movement disorders characterised mainly by a progressive spastic gait disorder and weakness of the legs, which results from a length-dependent axonopathy of corticospinal tract fibres. HSP type SPG11 is the most common recessive form and is caused by mutations in the SPG11 gene, which encodes the protein SPATACSIN. The pathophysiology of SPG11 is so far not well understood. In order to study the function of Spatacsin and to elucidate the key events leading to SPG11, Spg11 was disrupted in mice. Lack of Spatacsin in mice indeed causes a progressive gait disorder, which is paralleled by a progressive loss of cortical neurons and Purkinje cells. Degenerating neurons accumulate autofluorescent material. These deposits are associated with the lysosomal protein Lamp1, p62, a receptor for cargo destined to be degraded by autophagy, and with LC3, a marker for autophagosomes, suggesting that the deposits are related to autolysosomes. Distinct lysosomal functions, such as the processing of Cathepsin D and lysosomal pH do not differ between genotypes. Moreover, the levels of Lamp1 both in brain tissue and mouse embryonic fibroblast (MEF) lysates are not altered in knockout mice. Further supporting a defect in autophagic clearance, in knockout MEFs lipidated LC3 levels are increased and p62 levels are elevated in brain lysates. Consistently, the number of autolysosomes is increased while the number of lysosomes is decreased in KO MEFs. Upon starvation, the number of lysosomes decrease, but the lysosome number recovers upon sustained starvation only in wild-tpye MEFs. A depletion of lysosomes was also evident in vivo in Purkinje cells of 2 and 11-month-old knockout mice. These data suggest that owing to the decreased number of lysosomes, available for fusion with autophagosomes in knockout mice, autolysosomal clearance may be impaired which results in the accumulation of un-degraded material and finally causes death of particularly sensitive neurons like cortical motoneurons and Purkinje cells.